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URSI Asia-Pacific Radio Science Conference

Development of Control Sand RF Synthesis Chain for Absolute Quantum Gravimeter at TCG-CREST

Harish Rathore, Anju Mor, Aishik Acharya

Published Jan 1, 2025• 0 citations
Publisher

Abstract

Accurate and stable acceleration due to gravity measurements are critical for mapping spatial and temporal density variation of Earth's subsurface, precision inertial navigation, advanced metrology, detecting underground structures, and many other applications.For precise measurement of gravity, absolute quantum gravimeter (AQG) exploits atomic matter wave interference to achieve exceptional sensitivity (≤ 1 μGal) as well as offering longterm stability and mitigates of mechanical drifts.Gravity measurement with an AQG involves several processes such as cooling and trapping Rubidium-87 ( 87 Rb) atoms in a magneto-optical trap (MOT), followed by optical molasses and polarization gradient cooling (PGC), state preparation, atom-interferometry, and population detection.The operational sequences in these processes demand extremely precise event triggering and synchronized instrument operation with sub-microsecond accuracy.Since the sensitivity and accuracy of gravity measurements with an AQG are highly dependent on timing precision, any deviation in the sequence can introduce phase noise and systematic errors, ultimately affecting measurement fidelity.In this article, we describe a control architecture of an absolute cold atom gravimeter and the implementation of timing sequences.The design of a radiofrequency synthesizer is also described, crucial for ultra-fast switching and frequency/power control of laser beams in an AQG.

DOI
10.46620/ursiaprasc25/bahe8150

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•Jan 1, 2019•2 citations
Accurate and stable acceleration due to gravity measurements are critical for mapping spatial and temporal density variation of Earth's subsurface, precision inertial navigation, advanced metrology, detecting underground structures, and many other applications.For precise measurement of gravity, absolute quantum gravimeter (AQG) exploits atomic matter wave interference to achieve exceptional sensitivity (≤ 1 μGal) as well as offering longterm stability and mitigates of mechanical drifts.Gravity measurement with an AQG involves several processes such as cooling and trapping Rubidium-87 ( 87 Rb) atoms in a magneto-optical trap (MOT), followed by optical molasses and polarization gradient cooling (PGC), state preparation, atom-interferometry, and population detection.The operational sequences in these processes demand extremely precise event triggering and synchronized instrument operation with sub-microsecond accuracy.Since the sensitivity and accuracy of gravity measurements with an AQG are highly dependent on timing precision, any deviation in the sequence can introduce phase noise and systematic errors, ultimately affecting measurement fidelity.In this article, we describe a control architecture of an absolute cold atom gravimeter and the implementation of timing sequences.The design of a radiofrequency synthesizer is also described, crucial for ultra-fast switching and frequency/power control of laser beams in an AQG.
SummaryAbsolute quantum gravimeters (AQG) play a critical role in the precision measurement of gravitational acceleration (g). More and more compact, portable gravimeters are required for the real time applications. For their development, initial step involves preparing and trapping a cold atom cloud, which is the foundation of precision measurements. This work is part of the efforts being made at TCG CREST, India for development of AQG and current status on the magneto-optical trapping of87Rb atoms, and the characterization of the cloud's temperature is discussed.
Abstract The precision measurement of acceleration due to gravity (g) for potential scientific and industrial applications requires the use of absolute quantum gravimeters (AQGs) as quantum sensors. In this study, the development of magneto-optical trap (MOT) for the Rubidium-87 ( 87 Rb) atoms for integration into an atomic gravimeter has been discussed. The MOT serves as a critical component for the cooling and trapping of neutral atoms, providing a high flux, low temperature atomic ensemble for subsequent measurements in cold atomic physics, such as inertial navigation, exploration, and gravity mapping. Our setup emphasizes compactness without compromising atom number or temperature performance using the single 780 nm extended cavity diode laser. We have presented the optimized optical layout for efficient atom cooling, trapping, and detection. The obtained results show atom loading rate of ∼6.02 × 10 8 atoms within 221 ms that provides fast data acquisition for the entire cycle of AQG. The fluorescence signal from free falling MOT is also obtained, which provides time-of-flight of 211 ms and temperature of 57 μK. The system is robust against environmental perturbation and offers valuable insights into the advancement of atomic ensemble for the realization of high precision quantum sensors for geophysical and navigation applications.
At TCG CREST Kolkata, India, we have started the development of an absolute quantum gravimeter (AQG).In this paper, we will describe the present status and the development of a comprehensive physics package and optical setup highlighting its potential for precise gravity measurements with applications in mining, earthquake detection, and navigation.Our experimental setup has three main building blocks namely, sensor head, optical setup, and electronics.The paper provides insights into the status of our research, shedding light on achieved milestones, challenges, and promising outcomes.Additionally, it outlines the roadmap for future work, emphasizing ongoing developments and innovations.This study contributes to the advancement of measurement techniques for acceleration due to gravity, opening avenues for diverse practical applications and paving the way for further advancements in the field of cold atom gravimetry.
This paper reports recent remarkable achievements of cold-atom technologies and related operational devices in the area of Quantum Sensing and Metrology. We will present in detail the status of the Absolute Quantum Gravimeter (AQG) that has left the laboratory for geophysical studies. The AQG is an industry-grade commercial gravity sensor which today meets the objective to provide a gravimeter based on atom interferometry with laser-cooled atoms as a mobile turn-key device. We report on an operational stability of continuous absolute measurements of g at the 10-10 level in various types of environment during month-long continuous acquisition periods. The first unit of the AQG has traveled more than 7000 km, so we will comment on the last measurement campaigns and comparisons performed by the AQG. These have in particular validated the repeatability of the measurements at the 10-9 level, the ease of use and the robustness of such technology. This paper will also be the occasion to describe in more details the maturity of several key technologies such as intelligent integrated laser systems that can help Quantum Technologies with cold-atoms taking-off for a wider range of applications in Quantum Computing, Quantum Simulation and Quantum Communication.
Cold atom free-fall interferometers now realize state-of-the-art precision inertial sensors, opening perspectives towards embedded devices for applications and fundamental physics tests on earth and in space. The acceleration of gravity can be measured in a stimulated Raman transition interferometer where the acceleration under gravity of a cloud of freely falling atoms is sensed by three Raman pulses allowing to split, reflect and interfere atomic wave packets [1]. The gravimeter developped at SYRTE allows to measure the acceleration of gravity at state-of-the-art performances compared to the best classical absolute gravimeters, with a sensitivity of 2×10−8g over 1s and an exactitude of 5×10−9 g [2]. This exactitude is currently limited by wavefront aberrations of the lasers, sampled by residual transverse trajectories of the molasse-cooled cloud over the free fall. Unbalanced transverse trajectories will also bias the measurement due to Coriolis effect [3]. We will investigate the use of a colder atom source to reduce these systematic effects, by implementing Bose-Einstein condensation of the initial cloud, allowing both better position control and better focusing of our source.
Atomic interferometry is a highly precise metrological technique of interest to both fundamental and applied physics. One field in which atomic interferometry is of particular interest is gravimetry, where instruments based upon the controlled manipulation of atomic states have superseded the performance of classical devices. In such devices, a standing wave derived from laser beams with well-defined phase difference are used to impart momentum to atoms undergoing free fall, generating spatial superpositions along the axis of gravity, before returning the atoms to an overlapped final state for recombination. This thesis presents the design and experimental progress towards a gravity gradiometer based upon the interference of rubidium-87 atoms, cooled and launched in a fountain configuration. The fountain is leveraged from a decommissioned frequency standard, which has been re-purposed and redesigned for use as a combined absolute gravimeter and gravity gradiometer package. It is capable of loading 100 billion atoms in 100 ms and then cooling and launching these atoms in a moving molasses configuration, with a final temperature of 5 μK. Furthermore, the process can be quickly repeated, with a second cloud subsequently launched, reaching its apogee at the same time as the first cloud, allowing for the possibility of simultaneous measurement at different heights and, in the future, the inference of the gravity gradient. Control of the fountain is provided by a stable and miniaturised optical setup, allowing for use with a future transportable or field-based device. A Raman beam system to perform the interferometry sequence has been constructed and characterised, likewise designed in a minaturised and stable manner. The Raman beams simultaneously generated Ramsey interference fringes on two independent clouds of atoms -- one launched to a height of 0.7 m and the other to 1 m. Such fringes were clearly observable after 200 ms of evolution time. Mach-Zehnder interference fringes were also demonstrated for dropped and launched atoms, providing the capability for gravimetric measurements to be made once the system has been fully characterised. The principle causes of noise in the Mach-Zehnder cases were identified as arriving from a lack of atoms and the phase noise of the Raman beams. To overcome these challenges, a new Raman beam system has been preliminary designed and tested and a new iteration of the fountain, moving towards quantum degeneracy, has been planned.